1. Field of the Invention
The present invention relates to a receiver arrangement comprising at least two receive branches for receiving a wave whose frequency spectrum is practically constant in a predetermined frequency band, a combining circuit for producing a first combined signal by combining the waves at the output of the receive branches and a servo-control for adjusting a phase shifting circuit inserted in one of the branches for maintaining the spectrum of an output signal representing a combination of the waves of the two branches constant at the output of said arrangement, said servo-control comprising a frequency discriminating circuit for analyzing combined signals, an adjustable selective circuit for obtaining a frequency band of a combined signal as a function of the output signal of said discriminator and a control circuit for adjusting the phase shifting circuit so as to maximize the output signal of the selective circuit.
Such an invention finds important applications in space-diversity receive systems, more specifically, for microwave links. In this domain it has appeared essential to use means for combatting selective distortion effects which are caused by propagation. One of these means consists of appropriately combining the signals received from the same transmitter through two different aerials (space-diversity). The most widely-known way of acting consists of performing this operation with intermediate frequency. The two intermediate frequency signals are conveniently phase shifted and attenuated or amplified relative to each other so that the signal resulting from their summation (called combined signal) presents sufficient features for satisfying the conditions for a correct transmission. It has been known and proved that the combined signal can be and is in most cases better than the individual signals received by each of the branches. This is particularly noticeable in the case where one or the two received signals is or are affected by distortions due to a selective (frequency) propagation phenomenon. In effect, it is easy to show and verify that when the receive aerials are sufficiently interspaced (space-diversity) or directed according to different angles of incidence (angular diversity), the respectively received signals present uncorrelated distortions for which a convenient line combination is sufficient to restore a much less distorted and usable signal.
It is known to adjust the phase shifting circuit for augmenting the overall level (power maximization).
A more particular servo-control criterion is described in Japanese Patent Application No. 56-98035. In this Application the spectrum of the combined signal is analyzed and the phase shifting circuit is adjusted for equalizing the spectrum.
The servo-control consists of using a power maximizing arrangement of the power measured in a band of the combined signal spectrum which presents the least energy. Worded differently, it is supposed that the ideal spectrum presents a known envelope (feature of pseudo-random digital transmissions often used in microwave links) and that any difference relative to this envelope (distortion) is associated to a point of minimum energy in the band. This principle thus seems to belong to a criterion for band distortion minimization. In fact it has been proved and experimented that all prior-art digital signal demodulation systems are all the more sensitive to distortion if the total signal-to-noise ratio deteriorates, or worded differently, if the criterion of minimization of the distortions plays a leading part over the maximization of the power when distortions occur in the useful band.